Friday, March 2, 2012

30m OPERA TRX

There is a new mode on the market which was named OPERA. The interesting bit about it, it uses A1A modulation. This makes it suitable for LF and MF operations in The Netherlands, reason enough for me to have a further look.

It seems, the mode is used on HF and VHF too. There are frequencies mentioned for all the bands.
Since one has to start somewhere, 30m is the band of choice.

The OEPRA center frequency is mentioned to be 10.1365MHz. This frequency should immediatly ring a bell, at least if the dear reader is familiar with my blog and stuff I posted earlier about.

There is a standart crystal (and canned oscillator), which comes along quite handy for a subharmonic receiver: 5.0688MHz. This as an oscillator would result in a direct conversion LO of 10.1376MHz. Consequently, such a receiver would create an audio signal of 1.1kHz on the lower side band. Since the modulation is A1A, this does not matter at all. It is conceivable to add a crystal notch filter for the upper side band.

As to the transmitter, the solution is similarly simple. Just an oscillator using a 5.0688MHz crystal with a tiny downwards pull of 550Hz. That should be easily doable. Should I intend to build such a transmitter, I will use a VXO as to just cover the frequency range 10.1363-10.1369MHz, which, translated into oscillator range reads 5.068150-5.068450MHz, reflecting a VXO range of 300Hz.
Frequency doubling could be done either actively or passively by just two diodes.
The trusty old 74HC240 could serve as a driver or power amplifier.

Since the mode is keyed, both RX and TX can be easily tied together in a QSK fashion.
As a bonus, if the mode should become out of fashion once, the rig can easily converted into a QRSS receiver by changing the crystal notch filter to a normal crystal filter and pull the TX oscillator up.

In ITU region 2, this transceiver could be used for Feld Hell. Seen that the Hell frequency is rather close to the local oscillator frequency, it may be desireable to change the RX oscillator to 5.0680MHz. CMOS oscillators are available for this frequency.

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